Re: Question RE <Working Level>

[Charles Willis <caw@nrc.gov>]

Ivor:

The term "working level" was developed as a measure of the potential lung dose.  Physically, the lung dose is delivered by the decay products of radon, rather than by the radon itself.  In principle, under some conditions the radon concentration could be quite high but with a low concentration of radon daughters (e.g. as the result of filtration) so the dose potential would be low.  Conversely, it might be possible to have a high concentration of radon daughters with little actual radon present; in this case, the dose potential would be high.  In practice, the concentration of radon daughters generally is some fraction of the equilibrium value for the radon present; generally, the "fraction of equilibrium" is between about 10 percent and 100 percent.  This range is wide enough to make the radon concentration a poor measure of the potential dose.

The working level unit was devised as something that could be measured and various devices are available today for that purpose.

One working level corresponds to 100 pCi/L in 100 percent equilibrium; that is for Rn-222, the Rn-220 value is different.  It is a simple calculation if you remember that the 130,000 MeV/L is for the short-lived radon daughters (the radon alpha does not count nor does the long lived lead 210).  Of course, the decay chains must be recognized; e.g. lead 214 has no alpha but its decay product polonium 210 emits an important alpha.

Commonly, when radon levels are expressed as radon concentration, e.g. pCi/L, an equilibrium level of 50 percent is assumed more or less arbitrarily.

Relating working level to dose is difficult because the daughters are alpha emitters that are not actually inside the tissue and that are not uniformly distributed.  Deposition also varies with the attachment of the radon daughters to dust particles, and other things.  The "0.4 to 1.3" rad/WLM to the bronchial epithelium is about as good as you are going to get, although the actual variation probably is much greater than this. The dose number does not a help in a risk assessment, nor is it of value in an epidemiology study.

I would hate to think that a WLM to dose conversion factor would be based on a negotiation, but strange things happen.

WLM is a measurable quantity while dose is largely conjectural so epidemiological studies should be based on WLM.  What people actually do, however, is another matter.

Charlie Willis
caw@nrc.gov
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